Even though you have all the time in the world to plan your next move, sci-fi tactics game XCOM: Enemy Unknown still manages to be a nail-bitingly tense experience.

That's because one wrong move can practically doom your entire mission to eradicate a cell of aliens, deactivate a time bomb, or escort a foreign diplomat to safety.

One silly misstep could see a soldier dashing into a room full of ape-like Muton chargers, or outflanked by floating robot torsos, or in the sightlines of a bulbous-brained telepathic alien who can turn your medic into a mindless zombie.

Keep watching the skies

XCOM is an utterly ruthless game, and has no empathy when you screw up. Even your much-loved corporal, who's been with you since mission three and is loaded up with special traits and is named after your dog, is completely disposable.

And in Enemy Unknown, dead means dead. You'll have to hire a new rookie unit from flippin' Canada and start from scratch.

But failure is always your fault, and more thoughtful tactics would have saved your arse. You could position a sniper on top of a camper van as a lookout. You could lay down suppressing fire, or do an attack that forces an enemy to exit cover.

And when the stakes are this high, it's a dizzying, heart-thumping thrill when you pull off that successful operation. When you storm a pizzeria in Birmingham, and systematically eradicate a bunch of Sectoids with superior strategy? Boy - there are few feelings like it on iOS.

I want to believe

Whether you win or lose, your efforts will have a knock-on effect when you get back to base.

Between missions you'll have to look after a sprawling underground ant farm of labs, factories, satellite uplinks, and military barracks.

Your role as commander is to hire and fire soldiers, manage your funds, engineer new gear, scramble fighter jets to take out UFOs, and instruct the science team to research any tech you found - or shot - in your last skirmish.

You'll also have to play diplomat, as you juggle the demands of every member state in the XCOM project. Ignore China's pleas for help, or deny Mexico its request for a pair of laser sniper rifles, and that country might exit the project, and take its monthly funding with it.

They came from outer space

It's a complicated mix of bureaucracy and budget-balancing, and while you're given some direction by the game's small cast of B movie characters, you're mostly left to fend for yourself.

It's a terrifying amount of responsibility, and it's actually more likely that wrong moves in the base will screw up your game than wrong moves on the battlefield.

But this freedom lets you tackle the game at your own pace and in your own style, and getting a glowing monthly report card from your shadowy council overseer is a reward in and of itself.

Remember: we will be watching

So the game's great. But most of you already knew that, having played it on PC, Xbox 360, or PlayStation 3. The real question is: how does this iOS port stack up?

Well, the graphics are a downgrade. Compare it directly to, say, the PS3 edition and you'll notice low-resolution textures, far fewer of those dynamic camera angles, and flat lighting.

But it mostly affects the rare up-close-and-personal shots and in XCOM's natural habitat - a towering, isometric view of a graveyard or downed UFO or truck depot - it's an outstandingly detailed and intricate game.

There are a few glitches (try and position a camera inside a building without a few swear words), and a long loading screen at the start. But, hey, it's not the nightmare crash-fest that was 2K China's other mobile port, Civilization Revolution.

Is anybody out there?

The touch commands can be a pain. Try zooming out and you'll more likely spin the viewpoint by 90 degrees or set your cursor to a higher elevation. And zoom - accomplished by a classic iOS pinch - is the only function without a button alternative.

It is, in conclusion, an imperfect but completely playable port. It also has handy iCloud save sync and not even a whiff of an in-app purchase.

XCOM: Enemy Unknown is a terrific game. It's a smart and sophisticated mix of scrappy on-the-ground skirmishes and monster-scale world diplomacy, with enough variety that it doesn't fall into tactical fatigue.

Look past a few nuisance elements of this iOS port and you've got one of this generation's top strategy games on your telephone. Technology. It's weird.